1. Field of the Invention
The present invention relates to a double-wishbone type suspension system for an automotive vehicle wheel.
2. Description of the Related Art
This type of suspension system includes a hub carrier for supporting a wheel, whose upper and lower ends are rotatably connected to a vehicle body with upper and lower link devices, respectively. There are known various kinds of double-wishbone type suspension systems which are classified according to the manner of supporting the upper and lower link devices in the longitudinal direction of the vehicle body. One example of such a suspension system is disclosed in U.S. Pat. No. 4,556,238 or DE-A-3331247, which will be briefly explained below.
Referring to FIGS. 9 and 10, the known suspension system described above includes a hub carrier 22 for supporting the wheel 21, with its upper end 22a rotatably connected to a vehicle body at a predetermined position 23a through an upper link element 23. The hub carrier 22 has lower ends 22b, 22c which are rotatably connected to the vehicle body at predetermined points 24a, 25a through lower link elements 24, 25, respectively. These lower link elements 24, 25 form a lower link device together with a radius rod 26 which extends forwardly from a lower end 22d of the hub carrier 22. The radius rod 26 is connected to the vehicle body at a point 26a to support the lower link device in the longitudinal direction of the vehicle body. On the other hand, the middle portion of the upper link element 23 and the middle portion of the radius rod 26 are connected to each other by an integral link element 27 at positions 27a, 27b, to support the upper link element 23 in the longitudinal direction of the vehicle body and provide the suspension system with a wind-up stiffness.
However, in the case of the suspension system described above, the kingpin axis K.sub.3 passes through the connecting point 22a between the upper link element 23 and the hub carrier 22, as illustrated in FIG. 10. The position of the connecting point 22a itself is significantly restricted in view of limited installation space for the suspension device and the requirement to preserve a necessary suspension stroke, so that the degree of freedom in designing the setting of the kingpin axis K.sub.3 is extremely limited.
For example, the length of the upper link element 23 can not be shortened too much because the required suspension stroke has to be maintained, therefore the connecting point 22a is necessarily located at a position which is relatively outside of the vehicle body in its lateral direction. Therefore, the kingpin axis K.sub.3 passing through the connecting point 22a and the kingpin axis setting point of the lower link system tend to be oriented vertically. Accordingly, the point where the kingpin axis K.sub.3 intersects the outer circumferential surface of the wheel tends to be located relatively inside of the ground contact center of the wheel in the lateral direction of the vehicle body, generating a kingpin offset .alpha..sub.3 on the positive side.
On the contrary, in the case of a rear wheel suspension system, it is generally desirable for the kingpin offset to be generated on the negative side so that the toe angle variation caused by the braking force can be generated toward the toe-in direction to improve the stability of the vehicle. In the case of a front wheel suspension system, on the other hand, it is generally desirable for the kingpin offset amount to be substantially zero in order to prevent variation of steering reaction force during braking operation. The conventional suspension system, accompanied by a tendency of kingpin offset generation on the positive side as explained above, has difficulty to satisfactorily meeting the requirements imposed on both the front and rear wheel suspension systems, that is, that the kingpin offset not be generated on the positive side.
Furthermore, regarding the toe angle variation caused by the side force during turning of the vehicle, it is preferable for safety reasons that the kingpin axis provides a negative trail for a rear wheel, and a positive trail for a front wheel. Therefore, the variation of toe angle can be generated on the understeering side, that is, toe-in side for a rear wheel and toe-out side for a front wheel. However, for a conventional suspension system, to satisfy these conditions, there has been no alternative other than shifting the position of the upper link element either forwardly or rearwardly with reference to the longitudinal direction of the vehicle body.